1. Field of the invention
The present invention relates to an autofocus camera and, more particularly, to a motor drive lens shifting system for use in an autofocus camera.
2. Description of the prior art.
Generally, an autofocus camera includes a focus condition detector and a driving motor for driving a camera's objective lens. As the driving motor shifts the objective lens, the focus condition detector detects the focusing condition of an image of a target object to be photographed formed by the objective lens and produces a focus signal which designates in-focus condition, front-focus condition or rear-focus condition. When the focus signal designates in-focus condition, it means that the objective lens is so positioned as to focus the image of the target object sharply on a film plane. However, when the focus signal designates front-focus condition or rear-focus condition, it means that the picture taking lens is so positioned that the image of the target object to be photographed is out-of-focus. More particularly, when the focus signal designates front-focus condition, it means that the objective lens is sharply focused on a point in front of the target object, and when the focus signal designates rear-focus condition, it means that the objective lens is sharply focused on a point at the rear of the target object. The focus signal produced from the focus condition detector is fed to the driving motor to stop the lens shift when the focus signal designating the in-focus is produced.
According to the autofocus camera of the prior art, there is such a disadvantage that the lens is positioned more or less off from the actual in-focus position. In other words, the lens is set in the position which deviates from the proper focusing position. Therefore the image formed on the film surface by the prior art autofocus system would not be as sharp as the image formed by the manual focusing. There are a number of reasons that cause the deviation in the lens positioning, but mainly it is caused by the overrun of the driving motor and by the response time of the focus condition detector, as explained below.
According to the prior art autofocus system, the driving motor, when it is energized, runs very fast to incessantly shift the objective lens. Then, when the driving motor receives the focus signal designating the in-focus condition, the driving motor stops, not instantly, but with a deceleration. As the speed of the driving motor becomes fast, the deviation becomes great.
According to the focus condition detector of the prior art autofocus system, an array of light receiving elements aligned in a line, such as an array disclosed in U.S. Pat. No. 4,249,073, is employed. The array is coupled with a self-scanning type detector and a discriminator. The self-scanning type detector sequentially transfers light signal from each element in the array to the discriminator which then calculates the light signal so as to discriminate whether the lens is in-focus, front-focus or rear-focus. Thus, in the focus condition detector of the prior art autofocus system, it takes about from several 10 msec. to several 100 msec. for the signal transfer and calculation, and such a time is a considerably long time for the control of lens positioning.
In order to overcome the above disadvantage, it has been proposed to drive the motor in the opposite direction after having overrun. If the motor is driven in the opposite direction in the same speed as before, the motor overruns again, because of the same reason explained above, although it may be less than before. By carrying out this operation repeatedly, it is possible to locate the objective lens to the in-focus position without any deviation. However, such a control takes time and, accordingly, it is not appropriate for photography.